What this calculator does
QTc is the QT interval on an ECG, corrected for heart rate. The raw QT interval, the time from the start of the Q wave to the end of the T wave, naturally shortens as heart rate rises and lengthens as it slows, which makes the uncorrected number hard to compare between two people or between two readings taken at different heart rates. Bazett’s formula divides the measured QT by the square root of the RR interval (in seconds) to produce a figure that can be compared more consistently.
A QTc calculation is a piece of arithmetic, not a diagnosis. Whether a given QTc value is normal, borderline or concerning depends on age, sex, medications, electrolyte levels and the clinical picture around the reading, all of which are for a clinician to interpret. Use this to check the arithmetic on a QT and heart rate reading, and take the result to a doctor rather than acting on it alone.
The formula
Convert heart rate to an RR interval in seconds (60 divided by heart rate in beats per minute), take its square root, then divide the measured QT interval by that square root. Bazett’s formula is the most widely used correction, though it is known to over-correct at high heart rates and under-correct at low ones, which is one reason clinical interpretation matters more than the raw number.
| Term | Meaning |
|---|---|
| QT interval | The measured time from the start of the Q wave to the end of the T wave on an ECG, in milliseconds. |
| RR interval | The time between two consecutive heartbeats, in seconds, derived from heart rate. |
| QTc | The QT interval corrected for heart rate using Bazett’s formula: QT divided by the square root of the RR interval. |
The inputs explained
| Field | What to enter |
|---|---|
| Measured QT interval (ms) | The measured QT interval from an ECG reading, in milliseconds. |
| Heart rate (bpm) | Heart rate at the time of the reading, in beats per minute. |
When to use it
Reviewing an ECG report
An ECG report sometimes states the raw QT interval alongside heart rate without spelling out the corrected value; recomputing QTc directly checks that arithmetic before discussing the result with a clinician.
Monitoring a medication known to affect QT
Certain medications can lengthen the QT interval, and clinicians sometimes track QTc over time as part of monitoring; this is a way to check the calculation behind a tracked value, not to self-monitor in place of medical follow-up.
A nursing or medical education exercise
Working through Bazett’s formula by hand with real numbers is a common way QTc correction is taught before students see it calculated automatically by ECG machines.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does QTc change with heart rate at a fixed QT interval?
The same 400 ms QT interval, at a range of heart rates.
| Heart rate | QTc (Bazett) | RR interval |
|---|---|---|
| 50 bpm | 365 ms | 1.200 s |
| 60 bpm | 400 ms | 1.000 s |
| 70 bpm | 432 ms | 0.857 s |
| 80 bpm | 462 ms | 0.750 s |
| 100 bpm | 516 ms | 0.600 s |
| 120 bpm | 566 ms | 0.500 s |
Questions
What counts as a normal QTc?
Reference ranges differ by age, sex and the source consulted, and a single number without clinical context does not tell the full story. This calculator performs the correction; interpreting whether a given QTc is normal for a particular person is a question for a doctor.
Why correct QT for heart rate at all?
The QT interval naturally shortens at faster heart rates and lengthens at slower ones simply because each heartbeat takes less or more time, independent of any underlying issue. Correcting for heart rate removes that effect so QT readings taken at different heart rates can be compared meaningfully.
Is Bazett’s formula the only correction method?
No, other formulas exist (such as Fridericia’s), and different formulas can give somewhat different results, particularly at heart rates well above or below normal resting range. Bazett’s is the most widely used in routine practice despite its known limitations at extreme heart rates.
Should I be worried about my QTc result?
This tool only checks the arithmetic of a correction formula. A long or unusual QTc reading, or any symptoms alongside it, should be discussed with a doctor rather than interpreted from a calculator.
For a broader heart rate reference during exercise rather than an ECG reading, see the heart rate training zones calculator.